Arc flash is a common phenomenon in all the electrical enclosures as switchboards, switchgears, motor control centre, panelboard etc. Arc flash can be described as a short circuit through air that flashes from one live conductor to another conductor or to the ground, in which large amounts of light and waves of heat energy are released explosively from electrical equipment. The amount of heat produced by an arcing event in low and medium energy equipment can be very large. The electric arc produced by these sources can jump anywhere from a few inches to several feet, depending on the voltage level involved.
As electricity travels through body, literally seeking ground, it leaves a path of harm and destruction all along the path it takes inside of body. The high temperature during internal arc evaporates the nearby materials and heats up the surrounding air, which results in an overpressure in the switchgear compartment. Extreme pressure exerts a significant force on the enclosure structure such as frame, door, side panels, rear panels, roofs etc. and often dislodges these parts.
Most of the time the operator become the victim during the course of actions at workplace that involve electrical equipment. Arc flash can cause traumatic injuries. The superheated ball of gas accompanying the flash can cause burns over entire body and lung, even death from these injuries is a real possibility, since the human body is a great conductor of electricity. The hot gases and the transient pressure waves may endanger persons close to the switchgear and may seriously damage the electrical equipment and the building. In a typical arc resistant electrical enclosure, besides the mechanical stress to equipment, the danger to persons close to the electrical enclosure must be excluded. Here, after exceeding a predetermined pressure limit, the hot gases trigger over pressure relief devices and the enclosure structure should withstand the high pressure until it is released completely.
If an operator is working on a circuit breaker and say an arc event takes place then there would be minimum protection to him in the form of a door. The door can deform due to the pressure of arc and the flames and gases may affect the operator. The gases can also come out from the circuit breaker front face, sealing and sides of door. During an arcing event the high temperatures and pressure generated are sufficient enough to burn a Human Being and it also vaporize all materials in the vicinity of the arc.
In order to overcome the arc effects, various approaches have tried to make the existing doors rigid by improving the latches, hinges or by providing the gaskets to existing door to avoid gases coming out. In the above cases, gases and plasma may melt the gaskets or circuit breaker casing which may allow escaping of the gases. Additionally, a high intensity blast may bend the door making the gases escape. Also in all the cases the operator has to rack in or out without any protection as the circuit breaker will be protruding out of the door.
Another approach is tried to provide bellows to cover the circuit breaker. Even in this case the bellows may melt due to a high intensity blast again making it unsafe. Arc flash incidents are common and costly, and the frequency of reported accidents is increasing and the overall safety industry is concerned. The occurrence of arcing inside switchgear has many undesirable effects. The arc energy can cause sudden pressure increase inside the enclosure resulting in severe mechanical and thermal stress on the equipment particularly on doors.
The safety of operators against hot gases, radiation and fragmentation of the enclosure must be secured. The important acceptance criteria must be fulfilled for a successful internal arc test, e.g. correctly secured doors and covers do not open or indicators do not ignite due to the effects of hot gases. It is necessary to protect the operator working on the device from any accidents and also helping in redirecting the arc to proper ventilating paths, thus exhaling the gasses to the intended exhaust.
However, in all the above cases the operator is not completely safe and in all the cases the operator has to ensure that either he wears protective devices or he has to take extra precautions to avoid from the arc during a circuit breaker switching and racking operation. Therefore it is desirable to provide an improved protection system for the circuit breaker which overcomes all the disadvantages of the conventional protection system and to withstand the effects of an internal arcing fault, thereby protecting the operator from the accidents due to arcing effects.